changing look and feel is meant as a design tool, not as a runtime tool;
that is, Oracle recommends that applications should not allow the end
user to change the look and feel at runtime.

labels should always have an associated mnemonic, to allow navigation through
the keyboard (Alt+F, for example) instead of the mouse. Two
common exceptions are the default button (activated by hitting Enter or
Return) and the Cancel button (activated by hitting Escape) - these should
not
have an associated mnemonic.

choose mnemonics according to the typical
menu styles, if possible; otherwise, select the first letter, a prominent
consonant, or a prominent vowel, in that order of preference.

use the JLabel.setLabelFor method to associate a mnemonic with
an element which would not otherwise have one, for example a JTextField.

almost all short textual items (menus, buttons, labels, for example) should
have headline capitalization, where all words are capitalized except for
common words with up to three letters (a, an, the, and, or, so, yet, etc.),
and do not appear as the first or last word.

if the text is not short, then the capitalization used in ordinary prose
(sentence capitalization) should be used instead.